fix #xxx File names with multiple dots in atmapreader grunt task

There is a discrepency between the ``atmapreader`` grunt task and
``aria.core.DownloadMgr.resolveURL``.

When removing the extension, the grunt task is looking for the
first dot, whereas the ``DownloadMgr`` is looking for the last dot,
so that there is a problem for files containing multiple
dots (such as ``.tpl.txt``, ``.tpl.css``).

This pull request fixes the ``atmapreader`` grunt task to match the behavior
of the ``DownloadMgr``.